High-temperature resistive gas sensors based on ZnO/SiC nanocomposites

Increasing requirements for environmental protection have led to the need for the development of control systems for exhaust gases monitored directly at high temperatures in the range of 300–800 °C. The development of high-temperature gas sensors requires the creation of new materials that are stable under these conditions. The stability of nanostructured semiconductor oxides at high temperature can be enhanced by creating composites with highly dispersed silicon carbide (SiC). In this work, ZnO and SiC nanofibers were synthesized by electrospinning of polymer solutions followed by heat treatment, which is necessary for polymer removal and crystallization of semiconductor materials. ZnO/SiC nanocomposites (15–45 mol % SiC) were obtained by mixing the components in a single homogeneous paste with subsequent thermal annealing. The composition and microstructure of the materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The electrophysical and gas sensing properties of the materials were investigated by in situ conductivity measurements in the presence of the reducing gases CO and NH3 (20 ppm), in dry conditions (relative humidity at 25 °C RH25 = 0) and in humid air (RH25 = 30%) in the temperature range 400–550 °C. The ZnO/SiC nanocomposites were characterized by a higher concentration of chemisorbed oxygen, higher activation energy of conductivity, and higher sensor response towards CO and NH3 as compared with ZnO nanofibers. The obtained experimental results were interpreted in terms of the formation of an n–n heterojunction at the ZnO/SiC interface.


Introduction
The risk of air pollution is growing due to the development of new technologies in the chemical, metallurgical and food industries, the use of bio-fuels in the energy sector, modern waste treatment, and new automotive and aircraft engines [1,2]. Increasing requirements for environmental protection lead to the need for the development of control systems for exhaust gases that can directly monitor at high temperatures in the range of 300-800 °C. The composition of the main components of exhaust gas includes CO 2 , CO, SO 2 , H 2 S, NO x , C n H 2n+2 , and NH 3 . The ratio of these components depends primarily on the technology features and fuel type. High-temperature gas sensors are needed for local monitoring of pollution emissions, as well as for monitoring the complete combustion of fuel and controlling medium-temperature chemical and metallurgical processes [3][4][5]. The development of high-temperature gas sensors requires the creation of new materials that are stable at 300-600 °C, high humidity, and lack of oxygen. Nanostructured semiconductor oxides, such as SnO 2 , ZnO, WO 3 , and In 2 O 3 , that have been widely used in resistive gas sensors cannot be applied directly, primarily due to the drift of the sensor parameters at temperatures above 500 °C. The stability of nanostructured semiconductor oxides at high temperature can be enhanced by creating composite nanomaterials using highly dispersed silicon carbide (SiC). The unique physical and chemical properties of silicon carbide -wide band gap (E g = 2.4-3.2 eV), high Debye temperature 1400 K, high thermal conductivity of 4.9 W/cm·K, low reactivity to oxygen and water vapor -ensure the stability of composite materials with respect to temperature, radiation, chemical and mechanical effects [6,7]. It has been shown that in MO/SiC nanocomposites containing metal oxide (MO) and nanostructured SiC, the presence of silicon carbide inhibits the growth of MO crystallites at high temperatures [8]. The difference in the adsorption properties, reactivity and electrical behavior of semiconductor oxides and silicon carbide, as well as possible chemical interactions on their interface, cause changes in the sensor performance of composite materials. The SiC-based materials in the form of planar Pt/MO/SiC heterostructures were intensively studied as sensitive elements of field-effect or Schottky diode gas sensors. These materials have a high sensitivity to hydrogen and hydrocarbons in the temperature range of 200-600 °C [9][10][11][12][13][14]. The resistive-type sensors based on MO/SiC composite materials have not been practically studied. A few works of the MO/SiC composite material based on highly dispersed silicon carbide [8,15] showed the stability of the material structure at 600 °C and its high response to carbon monoxide.
Electrospinning is inexpensive tool widely used today for preparation porous, ultrathin fibers of SiC and metal oxides as well as MO/SiC composites from polymer solutions [16][17][18][19]. The combination of unlimited length, highly porous microstructure, and high surface area come together to create ideal gas sensor materials.
In this work, we prepared ZnO/SiC nanocomposite materials by mixing and heat treatment of electrospun ZnO nanofibers and nanocrystalline silicon carbide of 3C-SiC polytype. The effect of silicon carbide on the structure and electrical properties of composite materials was studied using different techniques. The work is aimed at creating the resistive-type gas sensors based on ZnO/SiC composites and studying the sensor performance towards the main components of the exhaust gases CO and NH 3 in air at a temperature range of 400-550 °C.

Results and Discussion
The nanocomposite synthesis scheme is shown in Figure 1 and described in detail in the Experimental part. The characteristics of the synthesized materials are summarized in Table 1.     creases with increasing silicon carbide content in the nanocomposites.
The study of the surface composition of the synthesized materials was carried out using FTIR and XPS methods. Figure 4 shows the IR absorption spectra of ZnO, SiC, and ZnO/SiC nanocomposites. The spectrum of zinc oxide contains an intense broad signal, corresponding to the stretching vibrations of Zn-O bonds (635-400 cm −1 ). The above spectrum also shows the signals from the multi-phonon vibrational modes of the ZnO lattice (990 and 870 cm −1 ). In accordance with the literature data, such oscillations do not appear at 78 K, but are noticeable at room temperature [20]. Nitro and nitrite groups [21,22] formed during the decomposition of PVP are also present on the surface of zinc oxide, as evidenced by the appearance of IR signals in the 1430-1260 cm −1 region, corresponding to the symmetric and asymmetric oscillations of the N-O bond. A broad peak in the region of 3750-3000 cm −1 is due to the stretching vibrations of hydroxy groups on the ZnO surface. The deformation vibrations of adsorbed water molecules are recorded at 1640 cm −1 . In addition to these, the spectrum contains peaks related to the vibrations of the C-O bond in CO 2 molecules adsorbed on the ZnO surface (2430-2320 cm −1 ) and C-H bonds (2920-2840 cm −1 ) in the residues of the organic components used in the synthesis of ZnO nanofibers.
The FTIR absorption spectrum of the SiC sample contains two intense peaks with absorption maxima at 900 cm −1 and 1067 cm −1 , corresponding to the stretching vibrations of the Si-C and Si-O bonds, respectively [23]. This indicates the formation of an amorphous SiO 2 shell on the surface of SiC nanoparticles, which does not appear on the diffraction patterns of the samples. In addition to these absorption lines, the spectrum contains the signals corresponding to O-H vibrations of surface hydroxy groups, deformation vibrations of adsorbed water molecules, and C-H bonds in the residues of organic components. All these oscillations are also present in the FTIR spectra of ZnO/SiC nanocomposites with the intensity ratio corresponding to the molar ratio of ZnO and SiC. Any additional vibrational modes do not arise in the FTIR spectra of ZnO/SiC nanocomposites.
To reveal the possible interactions between SiC and ZnO nanoparticles, and to shed light on the surface composition of the materials, we used X-ray photoelectron spectroscopy (XPS). Figure 5 and Figure 6 show the XPS spectra of ZnO, SiC and the ZnO/SiC_15 nanocomposite in the Zn 2p, O 1s, Si 2p, and C 1s binding energy regions. The survey spectra are provided in Supporting Information File 1 ( Figure S1). For the SiC sample, it was found that the Si 2p region contains three components at 100.6 (Si1), 103.0 (Si2), and 106.3 (Si3) eV ( Figure 5a). The first one corresponds to silicon carbide, while the second one refers to silicon oxide [24]. A weak third component may be associated with Si-O 2 bonds [19]. The formation of silicon oxide is also observed in the photoelectron spectrum in the O 1s region, containing two components at 532.9 (O1) and 536 (O2) eV. The first component is assigned to the oxygen bonded to two silicon atoms [24] while the second one may be associated with the chemisorbed oxygen (    The formation of nanocomposites is accompanied by a significant increase in the electrical resistance of the material, in comparison with ZnO nanofibers, over the entire temperature range studied ( Table 1). The resistance of SiC under these conditions is in the range of 10 9 -10 11 Ohm, which corresponds to the upper limit of the measurement range of the used setup. In the temperature range T = 400-550 °C, the conductivity of ZnO nanofibers and ZnO/SiC nanocomposites has an activation character (Figure 7). From the Arrhenius equation lnG = E a /k B T, where G is the material conductance, k B is the Boltzmann constant, the values of the activation energy E a were calculated. For ZnO nanofibers, E a = 0.40 ± 0.04 eV. This value lies within the error with the potential barrier at the grain boundaries eV s (the surface potential barrier energy between particles of nanocrystalline zinc oxide) determined by the method of temperature-stimulated conductance measurements [29,30] as eV s = 0.44 eV at T = 500 °C [31]. The creation of ZnO/SiC nanocomposites leads to an increase in the activation energy of conductivity up to 0.71-0.78 eV, and the value of Е a does not depend on the SiC content in nanocomposites (within the error, Table 1). The growth of the electrical resistance and E a can be associated with an increase in the concentration of surface oxygen species (confirmed by XPS), which form different acceptor levels at the ZnO surface and at the ZnO/SiC heterojunction.
The sensor properties of the synthesized materials were investigated by in situ conductivity measurements. Figure 8 shows the change in the resistance of ZnO nanofibers and ZnO/SiC nanocomposites with a periodic change in the composition of the gas phase in the presence of NH 3 (Figure 8a,b) and CO (Figure 8c,d) in dry air (Figure 8a,c) and at relative humidity RH 25 = 30% (at 25 °C, Figure 8b,d). In all cases, in the presence of a reducing gas, CO or NH 3 , a decrease in the material resistance is observed due to reaction of the target gases with the oxygen chemisorbed on the surface of n-type semiconductor materials: (1) where CO (gas) , NH 3(gas) are molecules of carbon monoxide and ammonia in the gas phase, is a particle of chemisorbed oxygen, e − is an electron released into the conduction band; CO 2(gas) , N 2(gas), H 2 O (gas) are the molecules of the reaction products desorbed from the surface of the material to the gas phase.
The data obtained allowed us to calculate the value of the sensor response as (3) where R air is the resistance of the material in background air, and R gas is the resistance of the material in the presence of the target gas (СО or NH 3 ). The temperature dependence of the sensor response is shown in Figure 9. The maximum sensor response of ZnO nanofibers is observed at operating temperatures in the range of 500-550 °C, and in the case of ZnO/SiC nanocomposites, at T = 450-500 °C for both reducing gases. In both cases, the formation of ZnO/SiC nanocomposites leads to an increase in the sensor response compared to the bare ZnO nanofibers. The nanocomposites ZnO/SiC_15 and ZnO/SiC_30 demonstrate the highest values of the sensor response. A further increase in the SiC content leads to an increase in resistance and a decrease in the sensor response of the nanocomposites. Thus, from the point of view of the measured values of the sensor response and the base resistance in the temperature range of  400-500 °C, the composition of the material, which corresponds to 15 mol % SiC, is optimal. An increase in air humidity up to RH 25 = 30% leads to an approximately two-fold decrease in the sensor response to CO and NH 3 .
The literature data characterizing conductometric gas sensors based on different MO/SiC systems are summarized in Table 2.
It should be noted that there are few examples found in the literature [32][33][34], and all the found sources consider different gases. This does not allow for a correct comparison of the sensitivity of the materials obtained in this work with the analogues described in the literature.
The observed effect of SiC on the sensor response of ZnO nanofibers toward CO and NH 3 should be considered within the framework of a model involving the formation of n-n heterocontacts at the ZnO/SiC interface [35]. According to a previous report [36] the conduction band minimum (CBM) of n-type ZnO lies 0.4 eV lower than the CBM of n-type SiC. The calculated band alignment of the wurtzite (2H) ZnO and SiC phases is presented in previous reports [37,38]. Taking into account the difference in the band gap (E g ) of 2H-SiC (E g = 3.3 eV [39]) and 3C-SiC (E g = 2.36 eV [40]) polytypes, and assuming that the position of the valence band for these polytypes does not vary significantly, we constructed a diagram of the band alignment for ZnO and 3C-SiC phases ( Figure 10). The estimated CBM position of wurtzite ZnO is 0.46 eV lower than that of 3C-SiC. The interface of an n-n junction transfers electrons into the lower energy conduction band [35]. The "accumulation layer" formed in this way can be depleted by subsequent oxygen adsorption on an enriched electron ZnO surface, increasing the potential energy barrier and enhancing the response formed due to reactions (Equation 1 and Equation 2). The decrease in the sensor response observed for all the samples with an increase in the concentration of water vapor in the gas phase may be due to the competition of oxygen and water molecules for the same adsorption centers on the ZnO surface [41].

Conclusion
ZnO/SiC nanocomposites based on ZnO nanofibers (wurtzite) and nanocrystalline SiC (3C polytype), obtained by the electrospining method, were investigated as sensitive materials for high-temperature resistive gas sensors. The introduction of SiC increased the sensitivity of ZnO nanofibers towards the reducing gases CO and NH 3 in the temperature range of 400-550 °C. This effect was accompanied by the increase in the activation energy of conductivity in this temperature range. The results obtained were interpreted in the context of the assumption of the formation of an n-n heterojunction at the ZnO/SiC interface, resulting in electron transfer from SiC to ZnO. The increase in the concentration of electrons in the near-surface layer of ZnO leads to an increase in the concentration of chemisorbed oxygen on its surface, which was confirmed by XPS. In turn, this determines an increase in the activation energy of conductivity and causes an increase in the sensor response of ZnO/SiC nanocomposites compared with ZnO nanofibers.

Materials synthesis
Nanocrystalline silicon carbide, SiC, and zinc oxide, ZnO, were prepared separately by electrospinning of polymer solutions followed by heat treatment in order to remove the polymer and crystallize the semiconductor material. The annealing conditions for polymer decomposition were determined by thermal analysis.

Fabrication of nanocrystalline SiC
Polycarbosilane (PCS) was used as a precursor. In a typical procedure, 1 g of PCS was dissolved in 10 mL of chloroform. After the PCS completely dissolved, 1 g of polyvinylpyrrolidone (PVP, M = 1 300 000) was added. The mixture was actively stirred for 5 h at 40 °C. The polymer solution was loaded in a plastic syringe with a metal needle (G21) with an internal diameter of 510 μm. The electrospinning was carried out at the conditions of 3 mL/h solution feed rate, with 150 mm distance and 6 kV voltage between the needle and metal collector. The formed fibrous tissue was collected in an alundum Al 2 O 3 crucible and annealed stepwise in an argon atmosphere at 220 °C (2 h, heating rate 1 K/min), 600 °C (2 h, heating rate 1 K/min), and finally at 1150 °C (6 h, heating rate 2 K/min). The obtained amorphous SiC was additionally annealed using the spark plasma sintering (SPS) method on a Spark plasma sintering system (LABOX-625) at a temperature of 1600 °C for 1 h under vacuum. As a result, 3C-SiC nanofibers with a cubic structure were obtained. The final annealing step was performed in air at 700 °C for 1 h to remove the rest of the carbon.

Fabrication of ZnO nanofibers
Zinc acetate (Zn(CH 3 COO) 2 ·2H 2 O) was used as a precursor. In a typical procedure, 200 mg of zinc acetate was dissolved in 10 mL of mixed (1:1) solvent composed of 2-methoxyethanol and isopropanol. After complete dissolution of zinc acetate, 900 mg of PVP was added and the mixture was actively stirred for 5 h at 40 °C. The electrospinning of the polymer solution was carried out at the conditions of 1 mL/h solution feed rate, with 125 mm distance and 12 kV voltage between the needle and metal collector. The fibrous material was collected and heated at 550 °С (5 h, heating rate 1 K/min) in air in order to remove the polymer and crystallize the ZnO.

Materials characterization
The phase composition was determined by X-ray diffraction (XRD) using a DRON-3 diffractometer (radiation Co Kα, λ = 1.7903 Å). The crystallite size (d XRD ) of SiC and ZnO phases in nanofibers was estimated from the broadening of the (100) ZnO and (111) 3C-SiC XRD peaks using the Scherrer formula. The measurements of the specific surface area (S BET ) and analysis of the porosity of the samples were carried out by the method of low-temperature nitrogen adsorption on an ASAP 2010 instrument (Micromeritics). Prior to this, all samples were evacuated at a temperature of 300 °C to 4 × 10 −1 Pa for 3 h. Based on the nitrogen adsorption isotherms obtained, the specific surface area, volume, and average pore size were calculated using BET (Brunauer-Emmett-Teller) and BJH (Barret-Johner-Halenda) models. The morphology of the nanofibers was studied by scanning electron microscopy (SEM) using a Carl Zeiss NVision 40 electron microscope with an intra-lens detector at an accelerating voltage of 5 kV. The IR spectra (FTIR) of the ZnO/SiC nanocomposites were taken on a Spectrum One (Perkin Elmer) spectrometer in transmission mode within the range 400-4000 cm −1 with 1 cm −1 steps. The XPS experiments were performed using an Axis Ultra DLD (Kratos) X-ray photoelectron spectrometer, equipped with a monochromatic Al Kα source. XPS spectra of core levels were fitted by Gaussian/Lorentzian convolution functions with simultaneous optimization of the background parameters. The background was simulated using a combination of a Shirley and a Tougaard background. The binding energies (BE) were corrected for the charge shift using the C 1s peak of graphitic carbon (BE = 284.8 eV) as a reference.
Gas sensor tests were performed by in situ conductivity measurements in an automatic set up with a flow chamber. The sensor resistance was measured at 1.3 V DC-voltage in situ under a controlled gas flow of 100 ± 0.1 mL/min at a temperature fixed in the range of 400-550 °C. Purified air with a preassigned humidity (RH = 0% and RH = 30% at 25 °C) was used as a background gas. The test gases containing CO (20 ppm) and NH 3 (20 ppm) were created from certified gas mixtures by the dilution with purified air with a pre-assigned humidity The corresponding gas flows were controlled by electronic massflow controllers (Bronkhorst).

Supporting Information
Survey X-ray photoelectron spectra of SiC, ZnO, ZnO/SiC_15 nanocomposite, and X-ray photoelectron spectra of ZnO/SiC_15 nanocomposite in the Si 2p region.
Supporting Information File 1 XPS data.